A fuel cell is a power generation system for producing electrical energy through a chemical reaction between oxidant and hydrogen or hydrogen contained in a hydrocarbon-based material such as methanol, ethanol, or natural gas.
A fuel cell can be classified into a phosphoric acid type, a fused carbonate type, a solid oxide type, a polymer electrolyte type, or an alkaline type, depending upon the kind of electrolyte used. Although each of these different types of fuel cells operates in accordance with the same basic principles, they may differ from one another in the kind of fuel, the operating temperatures, the catalyst, or the electrolyte used.
Recently, polymer electrolyte membrane fuel cells (PEMFCs) have been developed. They have power characteristics that are superior to conventional fuel cells, as well as lower operating temperatures, and faster start and response characteristics. Because of this, PEMFCs have a wide range of applications, such as for transportable power sources for automobiles, distributed power sources for residences and public buildings, and small power sources for electronic devices.
A PEMFC is essentially composed of a stack, a reformer, a fuel tank, and a fuel pump. The stack forms a body of the PEMFC, and the fuel pump provides fuel stored in the fuel tank to the reformer. The reformer reforms the fuel to generate hydrogen gas and supplies the hydrogen gas to the stack, where it is electrochemically reacted with oxidant to generate electrical energy.
Alternatively, a fuel cell may be a direct oxidation fuel cell (DOFC) in which liquid methanol fuel is directly introduced to the stack. Unlike a PEMFC, a DOFC does not require a reformer.
In the fuel cell system described above, the stack for generating the electricity has a structure in which several unit cells, each having a membrane-electrode assembly (MEA) and a separator (also referred to as “bipolar plate”), are stacked adjacent one another. The MEA is composed of an anode (referred to also as “fuel electrode” or “oxidation electrode”) and a cathode (referred to also as “air electrode” or “reduction electrode”) that are separated by a polymer electrolyte membrane.
The separators function both as channels for supplying the fuel and the oxidant required for a reaction to the anode and the cathode, as well as a conductor for serially connecting the cathode and the anode in the MEA or connecting the cathode of the MEA to the anode of a neighboring MEA. The electrochemical oxidation reaction of the fuel occurs on the anode, and the electrochemical reduction reaction of the oxidant occurs on the cathode and as a result of the transfer of the electrons generated by the oxidation/reduction reactions, electrical energy, heat, and water are produced.
As mentioned above, an MEA includes a polymer electrolyte membrane. The polymer electrolyte membrane functions as an electrolyte in the MEA. Commercially available fluoride electrolyte membranes such as a perfluorosulfonic acid ionomer membrane like NAFION® (fabricated by DuPont), FLEMION® (fabricated by Asahi Glass), ACIPLEX® (fabricated by Asahi Chemical), and DOW® XUS (fabricated by Dow Chemical) are often used for the polymer electrolyte membrane.
However, these commercially available polymer electrolyte membranes are known to have weak mechanical strength and to acquire pin holes upon long-term use, thereby causing the fuel to mix with oxidant so that energy converting efficiency and output characteristics are deteriorated. In order to compensate for the weak mechanical strength, a thicker electrolyte membrane is sometimes used, but this use of a thicker electrolyte membrane enlarges the volume of the MEA and increases the proton resistance and the cost of materials.
Further, the commercially available polymer electrolyte membrane may absorb water generated from the cathode of the MEA, thereby expanding its volume, and causing the polymer electrolyte membrane to peel away from the cathode (or anode) or increase the proton resistance of the interface between the cathode (or anode) and the polymer electrolyte membrane.